Flow Rate & Pipe Velocity Calculator
Calculate flow, average pipe velocity, or the inside diameter needed for a chosen velocity. Enter two known values, switch between US and metric units, and compare how pipe size changes the result.
THE FLOW CALCULATOR
Set the flow. Check the velocity.
Solve for flow rate, pipe velocity, or actual inside diameter. The example below shows 500 US GPM through a 6-inch inside diameter. Change the values to match your line.
Enter two values, then calculate to see pipe velocity and the equivalent units.
Compare velocity at different inside diameters
Holding flow at , a larger flow area lowers average velocity.
| Actual ID (in) | Velocity (ft/s) | Velocity (m/s) |
|---|
Illustrative actual inside diameters, not pipe schedule or SDR sizes. This table does not recommend a minimum or maximum slurry velocity.
Calculation basis: Q = vA for steady flow through a full circular pipe, using average cross-sectional velocity. For slurry, Q is the total mixture flow. This tool does not calculate pressure, friction loss, pump output, or minimum solids transport velocity.
UNDERSTAND THE NUMBERS
Flow rate and velocity answer different questions
Flow rate tells you how much liquid or slurry mixture passes a point in a given time. Pump duties are often stated in gallons per minute (GPM), liters per minute, or cubic meters per hour.
Pipe velocity tells you how fast that mixture moves through the pipe, usually in feet per second or meters per second. The two values are connected by the pipe’s internal flow area.
At the same flow rate, a smaller inside diameter produces a higher average velocity. Doubling the actual inside diameter makes the flow area four times larger, so velocity falls to one quarter of its previous value. That relationship is useful when comparing discharge hoses, checking an existing line, or preparing a pump duty specification.
One relationship. Three useful answers.
Use consistent units throughout. The calculator converts every input to SI units before solving, then converts the result back to your selected units.
01 / FLOW RATE
Q = v × A
Find the volume moving through the pipe from its average velocity and internal area. For a circular pipe, A = π × ID² ÷ 4.
02 / PIPE VELOCITY
v = Q ÷ A
Divide flow by internal area. This gives the average mixture velocity across the pipe, rather than the speed at any single point.
03 / INSIDE DIAMETER
ID = √(4Q ÷ πv)
Find the actual inside diameter associated with a specified flow and target velocity. Select a real pipe or hose by its documented ID.
Symbols: Q = volumetric flow; v = average velocity; A = internal cross-sectional area; ID = actual inside diameter. In SI units, use m³/s, m/s, m², and m. In US units, v (ft/s) ≈ 0.4085 × Q (US GPM) ÷ ID² (in).
FROM INPUT TO ANSWER
Four steps to a clearer pipe duty.
01
Choose the unknown
Select flow rate, pipe velocity, or inside diameter. The form shows the two inputs needed for that calculation.
02
Check the units
Use US or metric presets, or change an individual input unit. Switching units converts the entered value.
03
Enter actual values
Use inside diameter and operating mixture flow. In diameter mode, enter a project-specific target velocity.
04
Review the result
Check equivalent units and the diameter comparison. Copy the result for your project notes or pump discussion.
WORKED EXAMPLE / 500 US GPM
500 GPM through a 6-inch inside diameter
A discharge line carries 500 US GPM, and its measured or documented inside diameter is 6.000 inches. Convert the diameter to 0.1524 m and the flow to approximately 0.03155 m³/s.
The internal area is π × 0.1524² ÷ 4, or approximately 0.01824 m². Dividing flow by that area gives an average velocity of 1.7293 m/s, or 5.6736 ft/s.
At the same 500 GPM, an 8-inch actual ID reduces average velocity to approximately 3.1914 ft/s. This is a comparison of geometry and flow; it does not establish that either diameter is suitable for the material being pumped.
CALCULATED AVERAGE VELOCITY
5.6736 ft/s
1.7293 m/s
Flow: 500 US GPM
Actual inside diameter: 6 in
Equation: v = Q ÷ A
“6-inch nominal pipe” may have a different inside diameter. Use the manufacturer’s dimensions for your schedule, SDR, or hose construction.
How pipe size changes velocity at the same flow
Average velocity in a full pipe falls with the square of the inside diameter. The table shows the geometry for common actual IDs; it does not say which velocity suits your material.
| Actual ID (in) | 250 US GPM (ft/s) | 500 US GPM (ft/s) | 1,000 US GPM (ft/s) |
|---|---|---|---|
| 4 | 6.38 | 12.77 | 25.53 |
| 6 | 2.84 | 5.67 | 11.35 |
| 8 | 1.60 | 3.19 | 6.38 |
| 10 | 1.02 | 2.04 | 4.09 |
| 12 | 0.71 | 1.42 | 2.84 |
-44%
Going from a 6 in to an 8 in ID at 500 GPM drops velocity from 5.67 to 3.19 ft/s.
v (ft/s) ≈ 0.4085 × Q (US GPM) ÷ ID² (in). Values are full-pipe average velocity from geometry only. For slurry, check transport velocity separately. Use the calculator above for any other combination.
BUILT AROUND REAL PUMPING CONDITIONS
Slurry needs more than a flow number.
For a slurry line, enter the total mixture flow: carrier liquid plus solids. Dry tons per hour or cubic yards of excavated sediment per hour cannot be entered as GPM without additional density and concentration information. The calculated velocity is a bulk average; it does not mean every particle travels at that speed.
Velocity matters because solids may settle when transport conditions are unsuitable, while higher speeds can increase friction losses and wear. The appropriate operating range depends on particle size distribution, solids concentration, density, rheology, pipe geometry, and the material’s settling behavior. A single universal slurry velocity would hide those differences.
Use this calculator to describe the duty and compare pipe options. Then review the slurry characteristics, system head, and actual pump curve together. EDDY Pump’s slurry pumps and dredging equipment can be evaluated against those project requirements.
EDDY Pump excavator-mounted slurry pump attachment
TURN THE RESULT INTO A PUMP DUTY
Bring the complete duty into focus.
A flow calculation is one input to a pump selection. Bring these three sets of information together before choosing equipment.
Confirm the pipe ID
Pipe schedule, HDPE SDR, wall thickness, and hose lining affect the available flow area. Use actual internal dimensions, and check restrictions at reducers, couplings, and valves.
Calculate system head
The required flow must be delivered against the system’s head. Elevation, pipe friction, fittings, and pressure differences determine the duty the pump must meet.
Describe the material
For slurry, document particle size, concentration by weight or volume, mixture density, temperature, and viscosity or rheology where relevant. Identify debris and wear concerns.
Flow and velocity unit conversions
Pump data sheets, pipe specs and site reports rarely use the same units. These factors cover the conversions used on this page.
Flow (US gallons)
- 1 US GPM3.785 L/min
- 1 US GPM0.2271 m³/h
- 1 US GPM0.06309 L/s
- 1 US GPM0.002228 ft³/s
Velocity and length
- 1 ft/s0.3048 m/s
- 1 m/s3.281 ft/s
- 1 in25.4 mm
- ReminderUS gallons, not Imperial
Worked conversion: 500 US GPM is about 113.6 m³/h, 31.5 L/s or 1,893 L/min.
Need the head to push that flow through your line? Open the TDH Calculator →
Next Steps / Project Review
Put Your Flow Insights to Work
Use the result as a starting point for a more reliable project review. Confirm the values you entered, compare reasonable alternatives, and keep the assumptions visible for the next person who reviews the duty.
Review Your Inputs
Confirm the operating flow, actual inside diameter, and selected units. Check that the values describe the mixture and pipe condition you are evaluating.
Compare Scenarios
Test how changes in flow or actual pipe ID affect average velocity. Compare practical alternatives without treating the result as a complete sizing recommendation.
Document Assumptions
Record the source of each value, the unit system, pipe dimensions, and any material or operating conditions that could change the calculation.
COMMON QUESTIONS
Frequently asked questions
Practical answers about flow, actual pipe diameter, and what the result means for your system.
How do I calculate flow rate from pipe diameter and velocity?
Multiply the average velocity by the internal cross-sectional area: Q = v × π × ID² ÷ 4. Use the actual inside diameter and consistent units. Select Flow rate above, enter diameter and velocity, and the calculator returns the volume flow with equivalent units.
Can I calculate GPM from pipe diameter alone?
No. A pipe diameter gives you flow area, but not the speed of the liquid. You also need velocity, a measured volume over time, or enough system and pump information to determine operating flow. This calculator uses average velocity and inside diameter.
Is nominal pipe size the same as inside diameter?
No. Nominal size identifies a pipe size family. The actual ID depends on outside diameter and wall thickness, including schedule or SDR. Hose liners can also change the internal area. Use manufacturer dimensions or a reliable measurement rather than assuming the nominal size is the ID.
Does this flow calculator work for slurry?
It calculates bulk average mixture velocity from total mixture flow and flow area. It does not calculate solids settling, particle slip, slurry friction corrections, or a safe transport velocity. Those require material and operating data beyond the three values used here.
What is a good velocity for a slurry pipeline?
There is no single velocity that suits every slurry. A useful operating range depends on particle size, concentration, density, rheology, pipe diameter, and line geometry. The comparison table shows how diameter changes average velocity; it is not a pass/fail chart for slurry transport.
Why does a larger pipe reduce velocity at the same GPM?
A larger ID creates more internal area. At a fixed flow, the same volume moves through a larger cross-section, so average velocity decreases. Because area scales with diameter squared, doubling ID reduces velocity to one quarter. In an actual pump system, changing pipe size can also change the operating flow.
Does the calculator include friction loss, pressure, or pump horsepower?
No. It solves the relationship between flow, velocity, and area. Use the TDH calculator for an initial system-head estimate, then review the required duty against a pump curve. Pressure, power, and slurry transport corrections require additional inputs.
Are the results valid for partly filled pipes or open channels?
The calculator assumes a full circular pipe. Partly filled pipes and open channels have different wetted areas and hydraulic behavior. It also does not model compressible gas flow. The GPM unit used here is US gallons per minute, not Imperial gallons per minute.
What units does the flow calculator use?
Enter values using consistent units for flow, velocity, and inside diameter. The calculator converts the supported flow and length units and reports the result with equivalent units. For US customary inputs, GPM means US gallons per minute.
Calculation method and references
The calculator uses the continuity relationship Q = vA and the area of a circle. Calculations use unrounded SI values; displayed results are rounded. Diameter-comparison rows hold flow constant.
→ Hydraulic Institute: fluid flow, internal area, and average velocity
→ KSB Pump Lexicon: flow rate
→ Hydraulic Institute: solids and slurries